Generally, in a hydraulic construction machine such as a hydraulic excavator, a diesel engine is installed as a prime mover and a variable displacement hydraulic pump is driven by the engine to drive an actuator, thereby carrying out predetermined work. Engine control in that type of hydraulic construction machine is generally performed by setting a target fuel injection amount and controlling a fuel injector in accordance with the target fuel injection amount.
Also, control of the hydraulic pump is generally performed as displacement control in accordance with a demanded flow rate and as torque control (horsepower control) in accordance with a pump delivery pressure. In the torque control of the hydraulic pump, by decreasing the displacement of the hydraulic pump as the pump delivery pressure rises, an absorption torque of the hydraulic pump is controlled so as not to exceed a maximum absorption torque set in advance, thereby preventing an overload of the engine.
Speed sensing control disclosed in JP,A 57-65822, for example, is known as a technique for effectively utilizing output horsepower of an engine in the above-mentioned torque control of the hydraulic pump. The disclosed speed sensing control comprises the steps of converting a deviation of an actual revolution speed from a target revolution speed of the engine into a torque modification value, adding or subtracting the torque modification value to or from a pump base torque to obtain a target value of maximum absorption torque, and controlling the maximum absorption torque of a hydraulic pump to be matched with the target value. With the speed sensing control, when the engine revolution speed (actual revolution speed) lowers, the maximum absorption torque of the hydraulic pump is decreased to prevent stalling of the engine. As a result, the maximum absorption torque (setting value) of the hydraulic pump can be set closer to a maximum output torque of the engine and hence output horsepower of the engine can be effectively utilized.
Further, improved techniques of the speed sensing control executed in the torque control of the hydraulic pump are disclosed in JP,A 11-101183, JP,A 2000-73812, JP,A 2000-73960, etc. With those improved techniques, environment factors (such as an atmospheric pressure, a fuel temperature and a cooling water temperature) that affect the engine output are detected by sensors, a modification value of the pump base torque is obtained by referring to preset maps based on the detected values, and the maximum absorption torque of the hydraulic pump is modified in accordance with the modification value. Therefore, even when the engine output lowers due to environmental changes, the maximum absorption torque of the hydraulic pump is decreased by the speed sensing control under a high load condition to prevent stalling of the engine. At the same time, a lowering of the revolution speed of the prime mover caused by the speed sensing control can be made less and satisfactory workability can be ensured.